The invention relates to a table top with integral induction heating elements fitted into the lower side of the table top.
Several types of induction heating tabletops are known, e.g. vitroceramic top plates below which there are disposed a number of induction heating elements supplied with ELF power. These traditionally comprise a flat (xe2x80x9cpancakexe2x80x9d) coil parallel to the table top surface, and a number of radially disposed ferrite rods below which screen the radiation of ELF induction from going downwards and direct it towards the iron-containing cooking utensil above the coil. Such induction heating coil elements may also be disposed in a well in a very thick top plate, so that the distance between the coil and cooking utensil is not too large. Another solution is described in WO 94/05137, in which the coil is disposed in a core made of a type of densit concrete which is magnetically conductive and the whole assembly is cast into non-magnetic densit, forming a solid unit.
It has recently been recognized as attractive to dispose an induction heating coil element in conjunction with a material of nature, such as marble or wood. However, in particular marble is very brittle and the disposition of a well which will leave less than 5 mm of material immediately above the coil is a very complex undertaking which leads to breakage due to invisible cracks in the material of nature. Furthermore, the heating of the top surface due to transmission of heat from the cooking utensil back into the surface can lead to severe heat stresses, in particular when the cooking utensil is used as a skillet.
The above disadvantages are avoided in a construction according to the invention which is particular in that pole pieces which are cast in situ in a ferromagnetic particle containing material constitute a magnetic coupling between the cooking utensil and the induction heating element. In practice this means that the pole pieces perform the double function of reinforcing the table top, such as a marble slab, and that the required distance between the induction heating element and the cooking utensil may be obtained.
In an advantageous embodiment of the invention the pole pieces penetrate the surface to be flush with the table top. This means that the in situ casting of the pole pieces actually retain the parts of the table top which is inside the perimeter of the pole pieces. This has the added advantage that the delimitation of the active area above the induction heating element is clearly delimited and by a material which is similarly stone-like as the marble, i.e. the densit material.
In a further advantageous embodiment of the invention the pole pieces rise slightly above the surface. This has the advantage that a very intimate contact is obtained between the cooking vessel and the magnetic circuit, without any risk of scratching the cooktop surface. Furthermore the thin layer of air below the cooking vessel creates a heat insulating layer which prevents the heating of the cooktop by conduction from the cooking vessel.
The invention is not limited to the use of e.g. magnetic densit, indeed with less hard materials of nature it may be desirable to use a resin type casting material loaded with ferromagnetic particles.
In practice, the manufacture of a marble table top with an induction heating element with no visible markings will be performed by the steps of cutting a circular well in the reverse side of the marble slab which is supported firmly, by cutting a further central depression and a circular ring, leaving only a thin partition between the bottoms of the further depression and the ring and the front side of the marble slab, by casting a ferromagnetic particle loaded material in the depression and ring, by placing a induction heating element in the well and fixing it there.
In case visible markings are desired (and a magnetic structure closer to the cooking utensil), the cutting of a well may be dispensed with, and instead a circular saw is used to cut out a cylindrical plate which is sliced to the required thickness. The part of the cylindrical plate which formed part of the marble slab surface has a central depression shaped in it, and is centered in the hole obtained by the sawing, and magnetic densit is cast around it. The central depression is similarly filled with magnetic densite. An induction heating element is subsequently fixed in contact with the magnetic densite. The firm support of the marble slab for drilling and sawing may occur by means of a heat releasable wax composition adhering firmly to the top surface of the cooktop.
The invention functions best with an induction heating element of the kind comprising a core made of magnetic densit, but the advantages of the invention are also tangible with traditional induction heating elements which have only a series of radially disposed ferrite rods below the coil.